I bought the FlipKlip at the same time as the Paperbax holder, and I would have to say that the FlipKlip seems like a better product, at least for my uses. The FlipKlip is very small, but the design and the springs work very well at holding both narrow and wide books to their open pages. I've been using the FlipKlip on exercise machines at the gym for several weeks now, and it's never let me down. (The Paperbax is larger and therefore a bit less portable, and it's harder to turn pages with its design than with the FlipKlip.)
Buy FlipKlip Treadmill Book Holder 2-Pak!